New heavy uranium isotope made

Researchers using the KEK Isotope Separation System (KISS) in Japan have created a new isotope of uranium with 149 neutrons to go along with the element’s 92 protons (Phys. Rev. Lett. 2023, DOI: 10.1103/PhysRevLett.130.132502).

The researchers found the element as part of an effort to explore the different nuclei across the periodic table. Specifically, the researchers wanted to study neutron-rich actinide isotopes that creep toward what is known as the N = 152 shell gap. A nucleus with 152 neutrons is predicted to form a specific structure that should enhance nuclear stability.

To make heavy elements to study, the team, led by Toshitaka Niwase of the High Energy Accelerator Research Organization (KEK), fired a beam of uranium-238 nuclei at a platinum-198 target. This induced multinucleon transfer reactions, in which protons and neutrons flowed between the target and the projectile nuclei in both directions, and created new isotopes. The researchers could then weigh the isotopes to get their precise mass by using a special time-of-flight mass spectrometer attached to the element-creating apparatus. Among several previously created isotopes of plutonium, neptunium, uranium, and protactinium, the team also found a new nucleus, uranium-241.

Like all uranium isotopes, uranium-241 is radioactive and decays over time. But uranium-241 should stick around long enough for researchers to probe its properties. The researchers also say that combining multinucleon transfer reactions and time-of-flight mass spectrometry could be a new way to explore questions of nuclear structure and stability up to N = 152 and beyond.